As is well known, a waveform display apparatus such as a spectrum analyzer, a network analyzer, or the like displays the spectrum of a signal to be measured, the transfer characteristic of a circuit element to be measured, or the like while developing it on the frequency axis.
When a waveform is observed using a waveform display apparatus of such a frequency sweep type, it is required to be able to easily provide high-definition waveform observation for a user.
However, a conventional spectrum analyzer, network analyzer, or the like cannot satisfy the above-mentioned requirement in the present state. These problems of conventional techniques will be described below taking spectrum analyzer as an example.
In general, in a spectrum analyzer, when the spectrum of an unknown signal is to be analyzed and evaluated, the level and frequency of the spectrum displayed to be developed on the frequency axis must be observed.
However, an actually displayed spectrum resolution suffers from a limitation due to, e.g., characteristics of elements constituting the spectrum analyzer. More specifically, each spectrum pattern to be observed is not always displayed by a single line segment, but has a mountain-like pattern in which a spectrum upper portion forms a moderate curve, and its lower portion is spread, although it depends on a measurement condition. The level and frequency of a maximum level point (peak) of a spectrum display having a mountain-like pattern correspond to those of a spectrum to be observed. For this reason, the maximum point of the spectrum must be searched. When some spectra are present in a narrow frequency range, and their levels and frequencies are to be measured, each spectrum must be selected, and the maximum level on a display must be detected. Furthermore, in some cases, a spectrum may be displayed to have a valley-like pattern in a direction opposite to the above-mentioned case. In this case, the level and frequency of a minimum level point (dip) correspond to those of a spectrum to be observed.
In the spectrum analyzer for analyzing the spectrum in this manner, many functions allowing convenient analysis have already been added. Since the present invention provides some of these functions, various functions of the conventional spectrum analyzer will be individually explained in different items below for the sake of easy understanding of the characteristic features of the present invention.
1 Zone Marker
This function is disclosed in Japanese Patent Application, "Spectrum Analyzer" (Published Unexamined Japanese Patent Application No. 63-218869) by the same applicant (some common inventors) as the present invention. More specifically, in the zone marker function, in order to facilitate observation of a peak (dip) of a spectrum within a desired frequency range displayed on a display device, a desired frequency range in a measurement frequency region is set as a zone, the zone can be horizontally moved in the measurement frequency region, the zone width is also variable, and a peak (the top of a mountain) or a dip (the deepest bottom of a valley) is indicated by a marker. (See FIG. 25)
Thereafter, the same technique was disclosed as a U.S. Patent (U.S. Pat. No. 4,901,873).
2 Zone Sweep (Partial Sweep)
This function is disclosed in Japanese Patent Application, "Spectrum Analyzer" (Published Unexamined Japanese Patent Application No. 64-9371) by the same applicant (some common inventors) as the present invention. More specifically, in order to allow high-speed observation while maintaining original performance and functions of a spectrum analyzer, a signal is analyzed by analog sweep, so that variation states of a carrier wave and a signal adjacent to the carrier wave are displayed on a single screen to be easy to see, and components to be seen can be quickly observed. That is, in the zone sweep function, a narrow range adjacent to a signal of interest within a measurement frequency region is set as a zone (see FIG. 26), and only this zone range is repetitively swept. Spectrum data obtained by sweeping the narrow range is updated. However, data outside the zone (on the right and left sides of the zone shown in FIG. 26) are stored in a memory without being updated after they are obtained by a single sweep operation, and are displayed together. In this zone sweep function, the sweep range is narrowed to provide partial, high-speed performance.
Thereafter, the same technique was disclosed as a U.S. Patent (U.S. Pat. No. 4,839,583).
3 Signal (Center) Tracking
This function is used in spectrum analyzers commercially available from U.S. companies (Type 8568 available from Hewlet-Packard Corp., Type 2410 available from Tektronix Corp., and the like). These spectrum analyzers shift the frequency as the abscissa for each sweep, so that a peak point of a spectrum displayed on a screen of a CRT as a display is always located at the center of the screen. More specifically, every time a sweep operation is performed, a peak point is searched by signal (center) tracking, so that the frequency of the peak point corresponds to the center frequency (center) on the screen. (See FIG. 27)
4 Foreground (so-called FG) & Background (so-called BG) Two-frame Display
This function is disclosed in Japanese Patent Application, "Spectrum Analyzer" (Japanese Patent Application No. 2-15432; filing date Jan. 25, 1990) by the same applicant (some common inventors) as the present invention. A BG display indicates a wide-band sweep result (see a lower graph of FIG. 28), and when a desired signal within the sweep frequency range is selected by designating a zone (see the lower graph of FIG. 28), a bandwidth corresponding to the zone can be displayed as an FG display. The FG display is enlarged, as shown in an upper graph in FIG. 28. In addition, the zone designated on the BG display can be shifted, and the frequency which can be observed on the FG display can be changed in correspondence with a zone shift on the BG display.
Thereafter, the same technique was filed in U.S.A (U.S. Ser. No. 644,220; filing date Jan. 22, 1991).
5 Data Point Designation & Enlargement Function
As an early technique associated with a digital storage oscilloscope, "Digital Measurement Apparatus" (Published Unexamined Japanese Patent Application No. 50-6380) is known with techniques before this technique, all the several thousands of coordinate points on a display device do not have sufficient resolution for observation. The data point designation & enlargement function is developed in consideration of this situation, and includes an "apparatus for controlling addresses and display positions of data points to be displayed on a display device in response to selected data points and a selected enlargement coefficient" so as to display data at a limited number of coordinate points at a time (in other words, to enlarge and display the data).
Thus, in the above-mentioned techniques added to a conventional spectrum analyzer, some problems remain unsolved.
First, in the conventional technique 5, a function of enlarging an image to be observed is realized by selecting desired data points and designating an enlargement coefficient. However, in order to compensate for insufficient functions of this technique upon actual high-definition observation of a spectrum, techniques described in items 1 to 4 have been added according to requirements of users.
Of these techniques, the zone marker function 1 and the zone sweep function 2 will be examined below. When an observed spectrum drifts due to any cause, and falls outside the zone, the zone position must be set again. When a portion adjacent to an observed spectrum is to be enlarged, a user must instruct to change the center frequency (CENTER FREQ) by a panel operation so as to display the corresponding signal at the center of a screen of a CRT. (For example, an instruction for causing the frequency of the marker point to coincide with the center frequency must be issued.)
The signal (center) tracking function 3 will be examined below. In this function, only a portion inside a display range of a CRT screen is searched. For this reason, when a spectrum of a portion adjacent to a given signal is observed by sweeping a narrow band, the signal may abruptly drift, and may fall outside the CRT screen. In this manner, in order to search a signal in an observation disable state, a user must set a wider span (sweep frequency width) again to detect a signal to be observed, and thereafter, must narrow the span to restore an original state.
The foreground FG & background BG two-frame display function 4 will be examined below. In this function, in order to display two frame data, the display area on a panel surface of a device must be inevitably increased. That is, when the entire display area is suppressed to be reduced in scale, displayed data is not easy to see for a user. In addition, a user must set the zone position in the BG display again when a signal to be observed drifts. Furthermore, with this technique, spectrum data extends over two traces, and when data is saved or recalled, a large memory capacity is required.
On the other hand, in order to visually display spectrum components included in a measurement signal, a spectrum analyzer as shown in FIG. 29 is conventionally used.
In FIG. 29, a measurement unit P1 having a heterodyne receiver arrangement capable of sweeping a local frequency continuously sweeps and detects a predetermined frequency range of an input measurement signal, and outputs detection signals.
A waveform memory P2 updates and stores the detection signals output during one sweep operation as a series of waveform data for each sweep operation.
A display controller P3 displays waveform data stored in the waveform memory P2 on a display device P4 as a spectrum waveform to have the frequency axis as the abscissa.
A start frequency setting unit P5 sets a sweep detection start frequency of the measurement unit P1. A center frequency setting unit P6 sets a sweep detection center frequency. A frequency span setting unit P7 sets a width (span) of a sweep detection frequency.
A start.center frequency calculation unit P8 updates and sets a start or center frequency using a changed/set condition frequency with priority, so that the following relation can be established for condition frequencies from the frequency setting units P5, P6, and P7 for determining a range of the sweep detection frequency: ##EQU1##
For example, when the start frequency is changed/set while the frequency span is fixed, the start.center frequency calculation unit P8 calculates and updates/sets a center frequency which can satisfy equation (1) for the new start frequency and the frequency span.
Therefore, a spectrum waveform is shifted by a difference in start frequency, and the same applies to a case wherein the center frequency is changed/set.
When the frequency span is changed/set while the start frequency is fixed, the spectrum waveform is displayed in an enlarged or reduced scale to have the start frequency as the center.
When the frequency span is changed/set while the center frequency is fixed, the spectrum waveform is displayed in an enlarged or reduced scale to have the center frequency as the center.
Therefore, when a spectrum waveform as shown in FIG. 30A is displayed on a screen of the display device P4, if a spectrum near the center frequency (F(c)) is to be observed in an enlarged scale, the frequency span can be decreased while the center frequency is fixed. Thus, the spectrum waveform is displayed in an enlarged scale to have the center frequency as the center, as shown in FIG. 30B.
As shown in FIG. 31(A), when a spectrum near a point a is to be observed in an enlarged scale, the spectrum waveform is shifted (by changing the start or center frequency) so that the point a is located at almost the center frequency, and thereafter, the frequency span is decreased while the center frequency is fixed. Thus, as shown in FIG. 31B, the spectrum near the point a is displayed in an enlarged scale to have the center frequency as the center, and further detailed spectrum observation is allowed.
However, as an observation mode of a spectrum analyzer, adjustment of equipment or the like is frequently performed while alternately observing the overall spectrum and an enlarged spectrum of a portion of the overall spectrum. In this case, in the conventional spectrum analyzer shown in FIG. 29, as described above, if a portion to be observed in an enlarged scale is not moved to near the center frequency, when the frequency span is changed, a target waveform may fall outside a display range. In this technique, in order to restore the enlarged waveform to an original spectrum waveform, operations must be performed in a reverse order, resulting in inconvenience.
For this reason, a marker point which is arbitrarily movable on a waveform may be provided, as has been realized in an oscilloscope, and a function (zoom function) of locating the marker point at a display center upon operation of a special-purpose enlargement key, and performing an enlarged-scale display to have the display center as the center may be utilized. However, with this technique, an enlarged waveform is fixed at the display center, resulting in inconvenience.
In the spectrum analyzer shown in FIG. 29, every time a waveform is enlarged, the frequency span and the start frequency are updated/set. Therefore, in order to display an original waveform, complicated operations are required.
As a conventional apparatus which utilizes the above-mentioned signal (center) tracking function so as to visually display a spectrum component included in a measurement signal, a spectrum analyzer as shown in FIG. 32 is known.
In FIG. 32, the same reference numerals denote the same parts as in the spectrum analyzer shown in FIG. 29, and a detailed description thereof will be omitted.
More specifically, in FIG. 32, reference numeral P10 denotes a tracking unit for preventing movement of a spectrum waveform on a screen for a measurement signal suffering from a frequency drift.
The tracking unit P10 detects an address corresponding to a maximum value of waveform data stored in a waveform memory P2 from a peak position detection unit 11, causes a frequency difference detection unit P12 to obtain the difference between a frequency corresponding to this address and the center frequency, and shifts a sweep detection frequency range by the difference.
Therefore, when a spectrum waveform as shown in, e.g., FIG. 33A is displayed on a screen of a display device P4, and the tracking unit P10 is operated, the overall spectrum waveform is shifted, so that the position of a maximum-level spectrum A is located at the center of the screen (at the position of the center frequency), as shown in FIG. 33B.
Thereafter, even when this measurement signal suffers from a frequency drift, since the sweep detection frequency range is shifted to follow this drift, the spectrum waveform can be observed while the maximum-level spectrum A is fixed at the center of the screen.
However, in the conventional spectrum analyzer as shown in FIG. 32, when a spectrum ranging from a fundamental wave having a large level to high-order harmonics having small levels is observed on a single screen like in harmonic measurement, if the above-mentioned tracking function is operated for the measurement signal, the fundamental wave is fixed at the center of the screen, and a display range of harmonics is limited to a region half the entire screen, resulting in inconvenience.
In the conventional spectrum analyzer, when the level of a spectrum to be subjected to tracking is smaller than the level of other spectra, condition frequencies (the start frequency, the frequency span, and the like) must be adjusted in advance to cause a spectrum having a large level to fall outside the sweep detection frequency range. As a result, observation on a single screen is undesirably disturbed.
A conventional spectrum analyzer having an arrangement as shown in FIG. 34 is also known. FIG. 35A shows a display example measured by the arrangement shown in FIG. 34.
A case will be explained below wherein measurement of up to 5th-order (5f.sub.1) harmonic components of a signal to be measured whose fundamental wave f.sub.1 is at 100 MHz, as shown in FIG. 35A, is performed using the arrangement shown in FIG. 34.
A local oscillator la in a measurement unit 1 outputs a frequency-swept signal to a mixer 1b according to instructions from a control unit 10a and a sweep signal generation unit 11a, and causes the mixer to convert up to 5th-order harmonic components of an input signal to be measured into intermediate-frequency (IF) signals. Therefore, the local oscillator la continuously frequency-sweeps the input signal to be measured over a band of almost 500 MHz. The IF signals passing through a band-pass filter (to be referred to as a BPF hereinafter) 1c are detected by a detector 1d, are converted into digital data by an A/D converter 2, and are stored in a storage unit 3 in correspondence with the swept frequencies. Data stored in the storage unit (waveform memory) 3 are displayed on a display screen of a display device 4. The data display on the display screen is made on a coordinate system defined by the abscissa as a frequency axis and the ordinate as a level axis by a predetermined total number of dots, e.g., 500 points for each of the abscissa and the ordinate.
An analysis resolution, a display resolution, and a comprehensive measurement resolution will be described below.
1 Analysis resolution
The analysis resolution is an index representing performance capable of analyzing adjacent signals, and is expressed by the band itself of the BPF 1c. When a measurement is performed while improving the analysis resolution, i.e., narrowing the band of the BPF 1c, the measurement S/N is also increased.
In order to perform an optimal measurement by sweeping frequencies, a transient response with respect to the velocity of an IF signal passing through the band of the BPF 1c must be taken into consideration, and this relationship is given by the following inequality: EQU (RBW).sup.2 .gtoreq.K.times.BW/T (1)
where
T: sweep time
K: constant
BW: frequency sweep bandwidth
RBW: analysis resolution (bandwidth of the BPF 1c)
Note that BW/T represents the sweep velocity.
2 Display resolution of frequency axis (abscissa)
The display resolution is determined by the total number of dots on the abscissa, and the frequency sweep bandwidth.
More specifically, the display resolution=BW/(the total number of dots).
3 Example of numeric values:
If T=2 sec, BW=500 MHz, K=2, and the total number of dots=500,
analysis resolution=22.54 KHz
display resolution=1 MHz/dot
The numeric values in this case reveal that the comprehensive measurement resolution visually observed from the display screen is determined by the display resolution, and is 1 MHz. The analysis resolution=2.4 KHz is not effective.
In general, the comprehensive measurement resolution tends to be determined by the display resolution as the frequency sweep bandwidth BW is larger, and tends to be determined by the analysis resolution as the bandwidth BW is smaller although it depends on the sweep time.
In the conventional spectrum analyzer shown in FIG. 34, when only a portion near up to 5th-order harmonic components of a signal to be measured having a fundamental wave of, e.g., 100 MHz is to be measured, the following problems are posed.
1 Conventionally, since measurement is performed by continuously sweeping frequencies up to almost 500 MHz corresponding to the 5th-order harmonic component, the measurement resolution (or display resolution) near respective harmonic components of interest is low, thus often causing a measurement error.
For example, if a component other than a harmonic is present within the display resolution=1 MHz per dot in the above-mentioned numeric value example, the harmonic and other components are undesirably measured at the same time.
2 Since a band as wide as 500 MHz is measured, if there are many components other than harmonic components, an operation for specifying the harmonic components is necessary, resulting in inconvenience.
In order to solve the above-mentioned problems, a measurement device for making a display as shown in FIG. 35B is known.
This measurement device designates frequencies of respective harmonic components to measure the levels at the designated frequency points, and processes the data to display the data as a bar graph. In this measurement device, the above-mentioned problem 2 can be solved. However, when a signal to be measured which may include components other than harmonic components is to be measured, whether or not harmonic components are measured in practice cannot be confirmed.
As described above, in the conventional spectrum analyzers and their associated measurement techniques, high-definition waveform observation cannot be easily provided to a user, and it is an urgent subject to realize this in this field.